A wide range of respiratory disorders are characterized by periods of remission interspersed with periods of exacerbation. This group of disorders is known to have a reversible component to the disease processes which can be treated with a wide range of medications and ancillary therapies. These disorders range from obstruction of the upper airway, such as with seasonal allergy which can temporarily result in partial or complete blockage of the nasopharynx to certain types of sleep apnea which result in temporary partial or complete obstruction of the posterior pharynx during phases of the sleep cycle, to disorders of the trachea and bronchi (tracheomalacia, tracheal polyps and warts, and bronchitis) and particularly to disorders of the lower airways, such as asthma, cystic fibrosis and chronic obstructive pulmonary disease (COPD) which are characterized by inflammation and reversible bronchoconstriction. Exacerbations can run the spectrum from mild to life threatening and in many instances it is difficult for the patient, or in the case of a child, for the parent, to gauge the severity of the relapse.
Typically, physical examination by a physician and/or ancillary tests such as spirometry, pulse oximetry and arterial blood gases are used to gauge the degree of exacerbation. For some diseases, which occur periodically or during sleep, it is necessary to admit the patient to the hospital for formal and extensive testing to diagnose the etiology and severity of the disease. Patients with these disorders frequent emergency departments and physician's offices for diagnosis and treatment as it is difficult for them to gauge when a visit is appropriate and thus they consume a considerable amount of healthcare resources, often unnecessarily. Sleep apnea is the temporary absence or cessation of breathing during sleep, thereby causing oxygen to cease entering the body leading to hypoxemia (lack of oxygen in the blood) and often, for carbon dioxide (CO2) to accumulate in the blood (hypercarbia). In general, when there is lack of oxygen delivery due to sleep apnea, the oxygen saturation (SpO2), i.e., an amount of oxygen in the blood, decreases to an abnormally low level and CO2 can increase to abnormally high levels.
Sleep fragmentation during sleep apnea causes excessive daytime sleepiness (EDS) and hypoxemia during sleep. Chronic declines in oxygen saturation and increased CO2 may cause high blood pressure, arrhythmia, or other serious cardiovascular abnormalities. Occasionally, a decline in oxygen saturation and/or rise in CO2 may even have fatal results by causing a heart attack while a person is sleeping or increasing the likelihood while they are awake. It is reported that about 20 percent of the adult population of the United States suffers from snoring, and about 50 percent of those people that snore suffer from sleep apnea.
Children with sleep apnea display unique symptoms such as decreased attention span, erratic behavior, EDS, irregular sleep, rib cage retraction, and flaring of the ribs. Such children may do poorly in an academic setting and, in the most serious cases, may suffer from mental or psychological disorders. For infants or babies, sleep apnea may cause sudden death during sleep. Sleep apnea is typically classified into three main types: obstructive, central, and mixed. Obstructive sleep apnea is the most common form of sleep apnea and is characterized by a repeated closing of the upper airway on inspiration. Central sleep apnea occurs when the brain fails to send adequate signals to the diaphragm and lungs during sleep, thereby resulting in decreased respiration. Mixed sleep apnea is a combination of obstructive sleep apnea and central sleep apnea. Regardless of the type of sleep apnea, it results in a decrease in SpO2 and often retention of CO2. Interestingly, children may manifest only CO2 retention, without the classical finding of decreased SpO2. Thus, one of the major tools for diagnosing sleep apnea, pulse oximetry for measuring SpO2, may be of little value in diagnosing sleep apnea in children. A breathing disorder is clinically classified as sleep apnea when a cessation of breathing lasting for ten or more seconds occurs at least five times an hour or at least thirty times in a seven-hour period. Snoring is a sound made when a soft palate of the upper airway vibrates, and thus, is often a direct indicator of sleep apnea.
Polysomnography (PSG) is a test during which sleep architecture and function and behavioral events during sleep are objectively measured and recorded. See U.S. Patent Publication No. 2002/0165462. More specifically, a number of physiological variables, such as brain waves, eye movement, chin electromyogram, leg electromyogram, electrocardiogram, snoring, blood pressure, respiration, and arterial oxygen saturation, are measured extensively. At the same time, behavioral abnormalities during sleep are recorded with video tape recorders. Trained technicians and sleep specialists read the record to obtain comprehensive results about the severity of snoring, whether arrhythmia occurs, whether blood pressure increases, whether other problems are caused during sleep, and at what points the record differs from normal sleep patterns.
Full polysomnography is, however, quite labor intensive, requires considerable instrumentation and is therefore expensive to conduct. As a result, many sleep laboratories have found it difficult to keep up with the demand for this test, and long waiting lists have become the norm. Further, many patients find it difficult to sleep adequately when monitored and in strange surroundings. Given that obstructive sleep apnea (OSA) is quite prevalent, leads to serious complications and that treatment options exist, it is important that individuals suffering from the disease are identified.
A conventional full overnight PSG includes recording of the following signals: electroencephalogram (EEG), submental electromyogram (EMG), electrooculogram (EOG), respiratory airflow (oronasal flow monitors), respiratory effort (respiratory plethysmography), oxygen saturation (oximetry), electrocardiography (ECG), snoring sounds, and body position. These signals are considered the “gold standard” for the diagnosis of sleep disorders in that they offer a relatively complete collection of parameters from which respiratory events may be identified and SA may be reliably diagnosed. The RR interval, is derived from the ECG and provides the heart rate and arrhythmia recognition. Body position is normally classified as: right side, left side, supine, prone, or up (or sitting erect). Typically, the microphone and the body position sensor are taped over the pharynx. Each signal provides some information to assist in the visual observation and recognition of respiratory events.
Collapse of the upper airway is conventionally defined in PSG studies as when the amplitude of the respiratory airflow decreases by at least 50%, snoring sounds either crescendo or cease, and oxygen desaturation occurs. An obstruction event is confirmed (i.e., desaturation not an artifact) by the recognition of an arousal (i.e., the person awakens to breathe), typically identified by an increase in the frequency of the EEG, an increase in heart rate, or change in snoring pattern. The remaining signals assist in determining specific types of obstruction events. For example, the EEG and EOG signals are used to determine if an obstruction event occurred in non-rapid eye movement (NREM) or rapid eye movement (REM) sleep. The position sensor is used to determine if an airway collapse occurs only or mostly in just one position (typically supine).
A reduction or absence of airflow at the airway opening defines sleep-disordered breathing. Absent airflow for 10 seconds in an adult is defined as apnea, and airflow reduced below a certain amount is hypopnea. Ideally one would measure actual flow with a pneumotachometer of some sort, but in clinical practice this is impractical, and devices that are comfortable and easy to use are substituted. The most widely used are thermistors placed in front of the nose and mouth that detect heating (due to expired gas) and cooling (due to inspired air) of a thermally sensitive resistor. They provide recordings of changes in airflow, but as typically employed are not quantitative instruments. Currently available thermistors are sensitive, but frequently overestimate flow. Also, if they touch the skin, they cease being flow sensors. Measurement of expired CO2 partial pressure is used in some laboratories to detect expiration, but it is not a quantitative measure of flow.
In sum, the inventors have realized that conventional apparatuses and methods for diagnosing sleep apnea and other respiratory disorders have several disadvantages including being difficult to implement, being unable to detect all three types of sleep apnea, being unable to provide accurate and reliable results, and causing discomfort in a subject being monitored.